This invention relates to pumps and pumping methods. It especially relates to positive displacement screw pumps which are operated at high pump speeds in pumping low viscosity liquids, and various multi-phase fluid mixtures of liquids, gases, and solids especially in large volumes.
As used herein, high speed operation of a positive displacement screw pump means operation at speeds of at least 500 revolutions per minute (rpm), preferably at least 900 rpm, and most preferably at least 1200 rpm. It is anticipated that pumps disclosed herein can be operated at sustained speeds between about 1500 rpm and about 1900 rpm.
It is known to use positive displacement pumps, having counter-rotating twin screws, for pumping higher viscosity products, such as pastes, creams, oils and the like.
In the papermaking art, it is known to use pumps for pumping two and three phase media which are part liquid and part gas, for example foamed liquid containing 50 to 80 percent air by volume, which optionally include some solid material. Foamed fiber furnishes containing solid cellulosic fibers for use in papermaking processes are well known as disclosed, for examples, in U.S. Pat. No. 4,443,297 to Cheshire et al, herein incorporated by reference. Pumping of such multi-phase foam media has presented a plurality of problems as the pump speed, the volume of flow, and outlet pressure have been increased. Using conventional pumps, the increase in flow volume has not corresponded well with increase in speed of the pump because of the compressibility of the foamed media.
Accordingly, in order to achieve efficient pumping at higher volumes and higher pump speeds, applicants have found it expedient to use a positive displacement pump for pumping such foamed media. Such pumps have conventionally been used for pumping higher viscosity products, generally products which provide some fluid lubricity between the stationary pump casing and the moving impeller (e.g. screw).
An advantage of such positive displacement pumps is that they generally create isolated batches of the media being pumped, isolating the batches essentially at, or near, the inlet pressure, whereby batches of foamed media are susceptible to being pumped from the inlet to the outlet in essentially predictable volume, wherein the volume pumped, as measured at the pump inlet, is essentially linearly related to the speed of operation of the pump.
Applicants have found that, when pumping the above mentioned foamed media the pumping operation creates pressure pulses which are transmitted through the outlet of the pump. Essentially, the fluid in a isolated pumping cell is at a pressure below the outlet pressure of the pump. Upon reaching the outlet, the fluid at the outlet suddenly rushes into the open cell and compresses the fluid in the cell. The sudden rushing of the fluid into the newly opened cell causes a rapid, temporary, pressure change at the pump outlet. This pressure change is transmitted out of the pump, through the pump outlet, as a pressure pulse in the fluid in the enclosed pressurized system downstream of the pump.
While such pressure pulses are of little consequence in operations which comprise only transfer of the media, where the output of the pump is intimately connected with the formation of the web in a papermaking process, such pressure pulses directly affect the uniformity of flow of furnish onto the paper-making fabric, and accordingly, the uniformity, in the machine direction, of the paper so made.
Applicants have also found that some conventionally-produced screw type positive displacement pumps experience excessive rates of wear when operated for sustained periods at their rated speed of 900 rpm for pumping the above recited three phase media, containing about 1% to about 4% by weight cellulose fiber. For example, a typical such pump, having a designed clearance of 0.020 inch (0.5 mm.) between the rotating pumping screw and the stationary bore, had a measured clearance of 0.040 inches (1.0 min. ) after sustained operation for only three hours.
It is an object of this invention to provide improved screw pumps which can withstand high speed operation over extended periods of time with substantially reduced wear between the stationary and the rotating members.
It is another object to provide means to measure the wear of the pump parts without disassembling the moving member from the stationary member.
It is a further object to provide means to monitor the wear of the pump parts over time without disassembling the moving member from the stationary member.
It is another object to provide a method of monitoring the wear of the pump parts without disassembling the moving member from the stationary member.
It is still another object to provide pumps which can pump low viscosity fluids at high speed operation with lower amplitude pressure pulses.
It is yet another object to provide pumps which can pump low viscosity fluids at high speed operation with lower rates of change of pressure.
It is still another object to provide a method of pumping low viscosity fluids at high speed operation with lower amplitude pressure pulses.
It is a further object to provide a method of pumping low viscosity fluids at high speed operation with lower rates of changes of pressure.
It is yet another object to provide a method of balancing a screw for a screw pump without increasing the leakage between the higher pressure outlet end of the screw and the lower pressure inlet end of the screw, at the loci of removing material for achievement of balance, or otherwise reducing the pumping capacity of the screw over a typical 360 degree rotation of the screw; and without significantly weakening the screw.